Selenium sensitization of silver halide emulsions has been proposed for a variety of photosensitive systems. See, for example, Wood U.S. Pat. No. 3,531,289, issued Sep. 29, 1970, Hoyen U.S. Pat. No. 4,395,478, issued Jul. 26, 1983, Maskasky U.S. Pat. No. 4,435,501, issued Mar. 6, 1984, and Kofron et al. U.S. Pat. No. 4,439,520, issued Mar, 27, 1984. Selenium cyanide, in particular, has been used for chemical sensitization of silver halide materials. See Shepard U.S. Pat. No. 1,602,592, issued Oct. 12, 1926, McVeigh U.S. Pat. Nos. 4,408,196 and 4,407,197, issued Oct. 29, 1968, Dunn U.S. Pat. No. 3,442,653, issued May 6, 1969, Overmann et al. U.S. Pat. No. 3,736,141, issued May 29, 1973, and Japanese patent publication Nos. 50-071322, 50-071323 and 50-71324. In particular, the foregoing U.S. Pat. No. 3,442,653 describes sensitizing a silver halide emulsion with a noble metal and a labile selenium compound.
Chemical sensitization of silver chloride emulsions using sulfur and gold is well known. After the precipitation of photosensitive silver chloride grains in a gelatin medium is complete, it is a common practice to chemically sensitize the grains by addition of sulfur- and gold-containing compounds. These react with the surface of the AgCl crystals to improve the photographic performance of the film.
The effectiveness of sulfur and gold sensitization is limited under some circumstances. Normally, for a spectrally sensitized emulsion, the light sensitivity of an emulsion increases approximately in proportion to the surface area of the silver halide crystals. The present inventor discovered that the expected increase of sensitivity with increasing crystal size did not occur in an emulsion for the yellow layer of a color print film. In the case of the silver chloride emulsion in the yellow layer of a color print film, crystal size increases beyond about 0.35 micron edge length gave less speed than predicted. Beyond 0.45 micron edge length, the cubic crystals gave minimal further speed increases. This was a major problem because the film needs to have crystals of about 0.5 micron edge length with full sensitivity.